Federal and state regulations, for a variety of reasons, require that traffic information from airports without control towers be provided. Such information is useful in allocating funds, determining traffic use patterns, law enforcement, noise control, etc.
To date, there have been two primary methods for determining traffic at such airports, namely: (1) estimation of the traffic by the airport manager or (2) spot checking the traffic for a period of time. Both such methods have inherent drawbacks. First, if the information gathered is to be used to decide the allocation of funding, which is in turn dependent upon the volume of traffic, airport managers are not likely to want to deprive their airports of funds and may be prompted to provide the highest possible estimation of air traffic using their airport. Second, human surveyors of aircraft traffic at isolated airports are subject to distraction and inattention to their task, partly due to boredom. Also, human surveyors are commonly used for only "spot checking" which is inherently inconsistent and introduces error when the survey results are extrapolated to provide some type of total traffic count.
For spot checking over longer periods, or for continuous use, "automatic" counting devices such as those used on highways to count traffic can determine the total number of airplanes using a particular runway. Such devices, such as a "road-tube" laid across a taxiway at an airport where traffic is to be counted to record the event every time a vehicle runs over the road tube, include several drawbacks such as (1) aircraft land at a variety of positions on the runway and it may be that not every aircraft would run over the cable and (2) such simple counting methods provide only the total number of vehicles running over the road tube and provide no information as to the type of use the vehicle was making of the airport nor whether the vehicle was an automobile or an aircraft. Information such as, did the aircraft land, take off, or shoot a touch and go, directional information, and type of aircraft are not obtainable by such devices but such information is helpful in describing the overall traffic usage pattern at a particular airport.
Another "automatic" airport traffic monitoring device, which has been used for a number of years, comprises a recording device box, a cable, a battery, and a microphone located to one side of, and near the middle of, a runway. Particularly loud sounds, such as from an airplane taking off, trigger a sound recording device, such as a cassette tape recorder, which records a segment of tape for a period of a few seconds; an electromechanical counter is also advanced by one count. Hourly and semi-daily "beeps" are also recorded on the tape. The analysis of the airport traffic is accomplished by a person, trained in the art, who listens to the sound segments and decides whether it was an aircraft, with some information about aircraft type based on the quality of the sound. Barking dogs and fire engines are easily eliminated. The method requires extremely close attention because the sound segments are very close together in time, leading to fatigue and boredom for the sound interpreter.
The traffic analyzing device of the previously referenced application is capable of performing the desired monitoring, but was designed primarily for use at single runway airports. However, if used at a multi runway airport with one or more runways intersecting the runway to be monitored, spurious counts could be generated as a aircraft using the cross-runway past the intersection with the primary runway. The apparatus of this application is able to detect cross-runway use and exclude such use from the data taken with respect to the target runway through use of a multiple microphone array. In addition, if more positional information is required a system of two arrays may be used to track a target.